The present invention relates to a black toner, and more particularly, to a black toner using black composite particles having not only excellent blackness, fluidity and light resistance, but also an excellent dispersibility in binder resins, which toner also exhibits excellent blackness, fluidity and light resistance.
As recent image developing methods, there are mainly known a one-component developing method requiring no carrier, and a two-component developing method using both a black toner and a carrier. In the two-component developing method, the black toner is brought into frictional contact with the carrier in order to impart the black toner to an electrostatic charge having a signal reverse to that of an electrostatic latent image formed on a photosensitive member, cause the black toner to adhere onto the latent image by electrostatic attraction force therebetween, and neutralize the electrostatic charges having the opposite signals, thereby developing the electrostatic latent image into a visual toner image. As the black toner, there have been widely used composite particles obtained by mixing and dispersing black pigments such as fine carbon black particles in resins.
In presently predominant PPC-type copying machines, the black toner used in any of the developing methods has been required to have a good insulating property and/or a high resistivity. Specifically, the black toner has been required to have a volume resistivity value of not less than 1×1013 Ω·cm.
Also, it is known that the behavior of a developer in a developing device strongly depends upon the fluidity of the developer which further influences a frictional electrification property between the black toner and the carrier in the two-component developing method or an electrification property of the black toner on a sleeve in the one-component developing method. Thus, with the recent tendency toward high image qualities such as high image density and excellent tone gradation as well as high speed of the developing device, it has been strongly required to enhance the fluidity of the black toner.
Further, with the recent tendency toward reduction in particle size of the black toner, it has been more strongly required to improve the fluidity of the black toner.
As to this fact, at page 121 of “Comprehensive Technical Data for Development and Utilization of Toner Materials (1985)”, it is described that “With the wide spread of printers such as IPC, printed images have been required to have a high quality. In particular, it has been required to develop high-definition and high-accuracy printers. As is apparent from Table 1 showing a relationship between various toners and definitions of images obtained, the wet toner having a smaller particle size can realize a higher definition of images. Also, in order to enhance the definition of images obtained using a dry toner, the reduction in particle size of the toners is similarly required. . . . As to toners having a small particle size, it has been reported that the by using a toner having a particle size of 8.5 to 11 μm, the generation of fog in background area can be inhibited and the amount of toner consumed is reduced. Further, by using a polyester-based toner having a particle size of 6 to 10 μm, there can be become a high image quality, stable electrification property and prolonged service life of the developer. However, such toners having a small particle size have many problems to be solved upon using, such as productivity, sharpness of particle size distribution, improvement in fluidity . . . or the like.”.
In addition, it has been required that the black toner can exhibit linear and solid-area copy images having high blackness and high image density.
As to this fact, at page 272 of the above “Comprehensive Technical Data for Development and Utilization of Toner Materials”, it is described that “. . . although the development using a powdery toner is characterized by high image density, not only fog concentration but also image properties as described later are largely influenced”.
Further, recording papers having images printed with the black toner are usually preserved for a long period of time after printing. Therefore, the black toner is required to have an excellent light resistance in order to keep clear printed images.
As described above, the black toner has been strongly required to be improved in various properties thereof. In particular, it is known that black pigments exposed to the surface of the black toner considerably influence developing characteristics of the black toner. Thus, various properties of the black toner have a close relationship with those of the black pigments mixed and dispersed in the black toner.
Namely, since the fluidity of the black toner largely varies depending upon the surface conditions of the black pigments exposed to the surface of the black toner, the black pigments themselves have been strongly required to exhibit an excellent fluidity. Also, the degrees of blackness and density of the black toner largely vary depending upon those of the black pigments contained in the black toner. Further, the light resistance of the black toner largely varies depending upon that of the black pigments contained in the black toner.
As the conventional attempt for improving the blackness of non-magnetic particles, there is known a method of forming a coating layer comprising organosilane compounds obtainable from alkoxysilane compounds or polysiloxanes, on the surface of respective hematite particles or iron oxide hydroxide particles, and then forming carbon black coat on the coating layer (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-338191(1999) and 2001-13730). Also, as the conventional attempt for tinting magnetic particles, there are known a method of coating the surface of the respective magnetic particles with a colorant through a coupling agent (Japanese Patent Application Laid-Open (KOKAI) No. 60-26954(1985)); a method of tinting the magnetic particles with dyes (Japanese Patent Application Laid-Open (KOKAI) No. 59-57249(1984)); or the like.
At present, it has been strongly required to provide a black toner exhibiting excellent blackness, fluidity and light resistance. However, conventional black toners have failed to satisfy such properties.
That is, in the case where the known fine carbon black particles are used as black particles for the black toner, the amount of the fine carbon black particles used in the black toner must be limited to a certain low level in order to obtain such a black toner having a volume resistively value as high as not less than 1×1013 Ω·cm. As a result, there arises a problem that the black toner is insufficient in not only blackness but also fluidity.
Also, in Japanese Patent Application Laid-Open (KOKAI) Nos. 11-338191 (1999) and 2001-13730, there are described the black non-magnetic particles obtained by forming the coating layer comprising organosilane compounds obtainable from alkoxysilane compounds or polysiloxanes, on the surface of the respective hematite particles or iron oxide hydroxide particles as core particles, and then forming carbon black coat on the coating layer. However, since the hematite particles and the iron oxide hydroxide particles have a specific gravity as high as 4.3 to 5.5 and the amount of carbon black adhered onto the surface of the core particles is as small as about 25 parts by weight based on 100 parts by weight of the core particles, the obtained black non-magnetic particles also have a high specific gravity, thereby failing to obtain black particles for black toner having a low specific gravity, for example, not more than 3.5. Further, since the hematite particles and the ferric oxide hydroxide particles are insufficient in fluidity, the toner obtained using such particles also fails to exhibit a sufficient fluidity.
In Japanese Patent Application Laid-Open (KOKAI) No. 60-26954(1985), there are described the magnetic particles coated with a colorant. However, it is difficult to completely shield and hide the hue of the magnetic particles as core particles, thereby failing to obtain a black toner having a high blackness.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 59-57249(1984), there are described the magnetic particles tinted with dyes. However, since the dyes are used as a colorant, the obtained particles fail to show a sufficient light resistance.
As a result of the present inventors' earnest studies, it has been found that by using black composite particles having an average particle diameter of 0.005 to 0.30 μm and exhibiting excellent blackness, fluidity and light resistance, which comprise extender pigments as core particles, a gluing agent-coating layer formed on the surface of the core particle and a black pigment coat uniformly formed on the gluing agent-coating layer, the obtained black toner can exhibit excellent blackness, fluidity and light resistance. The present invention has been attained based on the above finding.